Personally, I think the absolute best time to take PPB is with AOW. IN AOW I teach the PPB dive first. Then I leave time for buoyancy practice in dives 2 through 5. Finally we sum in all up with dive 6 the last PBB dive. It’s amazing how good you get at buoyancy with 6 dives of practice. I recommend taking AOW/PPB after about 10 to 15 dives after OW. This gives you a chance to practice all the OW skills to the point you can free your mind to focus on the new skills.
The fundamental problem new divers face regarding buoyancy is best resolved with practice. New divers don’t fully trust their equipment and their skills yet. This results in the new diver subconsciously wanting to keep a lot of air in their lungs to feel safe making them unnaturally buoyant.
The result is during open water the OW instructor unnecessarily piles on the weight because he is much more afraid on a runaway ascent then teaching good buoyancy. There is more chance for injury and law suit with the runaway ascent case then letting the new diver bounce off the bottom kicking up silt, which is what happens in Texas. As an instructor I am always taking weight off on students.
What to do? Practice relaxing. First thing is to practice a slow deep breathing pattern. I like to exhale fully, pause for three seconds, inhale fully pause for three seconds. Notice I said pause and not hold your breath. They are very different. When you pause your air way is still open. The pauses allow time for good gas exchanges (O2 and CO2) between blood and air. This also relaxes you.
Practice ascending with your breath. Inhale fully pause three seconds, exhale half, inhale fully and repeat. Practice descending with you breaths. Exhale fully, pause three seconds, inhale half, exhale fully and repeat. If you can’t do this without losing control, you have too much weight and too much air in your BC to accommodate the weight. Drop some lead and keep practicing.